1. FIELD OF THE INVENTION
The present invention relates to a relative distance indicating speedometer for a vehicle for measuring the relative distance of a vehicle carrying the speedometer with respect to an object such as the vehicle in front and indicating the measured relative distance in the speedometer.
2. DESCRIPTION OF THE PRIOR ART
In the past, it has been the practice for the driver of a vehicle to visually perceive the relative distance of his vehicle with respect to an object which is in front thereof, so that whether or not his vehicle is running at the proper speed is judged and the vehicle speed is controlled accordingly. In this case, the perception of relative distance by the driver as well as his judgement whether the speed of his vehicle is proper or not are conventionally performed only with a low degree of reliability, and this is known to be a cause of accident such as rear-end collisions. In an attempt to prevent the occurrence of such accidents, a driving assistance system has been proposed which comprises a relative distance detector for measuring the relative distance of a vehicle with respect to the vehicle in front, a relative speed detector for detecting the relative speed of the vehicle with respect to the vehicle in front, a vehicle speed detector for detecting the speed of the vehicle itself, and a computing control unit for operating on the relative distance, relative speed and vehicle speed detected by the respective detectors and giving a warning and performing the required automatic control action. A disadvantage of this type of systems is that when the existence of any dangerous condition has been determined by the computing control unit, it gives a warning and performs the required automatic control operations all at once with the resulting danger of mentally disturbing the driver and thus causing him to misoperate the vehicle. Another disadvantage is that since any malfunction of the system itself has the danger of causing a very dangerous situation, the component parts of the respective detectors must have a very high degree of precision and moreover these component parts must be constantly checked and adjusted thus making the system very expensive to manufacture.